1. Field of the Invention
The following invention relates to controls for variable-air-volume (VAV) heating, ventilating, and air-conditioning (HVAC) systems, specifically to control of a supply fan in VAV HVAC systems.
2. Description of the Prior Art
Modern buildings typically have complex heating, ventilating, and air-conditioning systems to control indoor temperature, pressure, ventilation rate, and other variables in a way that makes efficient use of energy. One way to conserve energy in these systems is to use a so-called variable-air-volume design. Key components of a variable-air-volume system are a supply fan and terminal units. The supply fan is a prime mover that causes air to move. A terminal unit contains a throttling damper that regulates an amount of air supplied to a space in a building that it controls in order to regulate temperature and ventilation in that space.
In a variable-air-volume system, the flow rate of conditioned air supplied to a building is adjusted so that no more air than necessary is used. Variable flow is achieved using controls on or near the supply fan and by the use of controls on the terminals. The supply fan controls adjust the speed of the fan, an angle of the fan blades, an angle of guide vane at an inlet or outlet of the fan, or by adjusting a damper upstream or downstream of the fan that throttles the flow. The controls on the terminals determine how much air flows through each terminal.
The most common control strategy for the supply fan of variable-air-volume systems is to regulate a static pressure in a supply duct at a point downstream of the supply fan. This strategy seeks to keep the static pressure at a measurement point constant at all times. Control strategies based on a constant static pressure in the supply duct have been proposed in U.S. Pat. No. 4,437,608 to Smith (1984) and U.S. Pat. No. 6,227,961 to Moore et al. (2001). U.S. Pat. No. 4,836,095 to Wright (1989) describes a variant of this strategy for systems that have multi-speed fans rather than fans in which the speed is continuously variable. A rule of thumb for this strategy is to locate the pressure sensor two-thirds of the distance from the supply fan to the end of the supply duct. A problem with this strategy is that it is inefficient at part-load conditions, when the supply flow rate is significantly lower than a design flow rate, which is the flow rate at which the system should operate when the fan is running at full speed.
A control strategy that overcomes the problem of constant static pressure control is one in which a static pressure setpoint is reset based on a position of a terminal damper that is most open. Control strategies that reset the static pressure based on the position of the terminal damper that is most open have been proposed in U.S. Pat. No. 4,630,670 to Wellman and Clark (1986) and U.S. Pat. No. 5,863,246 to Bujak (1999). An objective is to keep this damper nearly open or completely open. Doing so reduces throttling losses at part-load conditions.
One problem with resetting static pressure based on the position of the most-open terminal damper is that it requires that the control system be able to measure the position of every terminal damper. Large systems could have hundreds of terminal dampers. Requiring terminal damper position measurement adds cost to the HVAC system.
Another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a communications failure. The terminal dampers are usually located far from the supply fan, so a digital communication network is used to connect the terminal unit control device, which knows the terminal damper position, with the supply fan control device. A failure in the network connecting these devices will cause the control strategy to fail.
Yet another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a terminal unit failure. If even one of the terminal units is not working properly the resetting strategy will not work properly.
Still another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a design flaw in which one or more terminal dampers is undersized. In this case the undersized terminal damper will require high pressure to achieve its required flow, causing large throttling losses at the terminal dampers that are not undersized.
A final problem with resetting static pressure based on the position of the most-open terminal damper is that it is difficult to tune. The most efficient operating point is when the most-open damper is completely open. If the strategy tries to keep the most-open damper completely open then the strategy cannot determine if the supply duct pressure is too low. If the controller tries to keep the most-open damper nearly completely open, then when it becomes completely open due to a disturbance in the system, the strategy cannot determine if the pressure is just slightly too low or far too low.
Several variants of static pressure resetting have been used. For example, one strategy resets the static pressure based on an average position of a set of terminal dampers that are most open. The averaging feature allows this strategy to reduce the energy consumption at part load conditions even if a small number of terminal units fail or are undersized. However, the strategy is still limited by the need for terminal damper position sensing, is still sensitive to network failure, and is still difficult to tune.
More complex strategies for controlling supply fans have been proposed in U.S. Pat. Nos. 5,540,619 and 5,573,181, both to Ahmed (1996). These inventions require the measurement of flow or pressure in all branches downstream of the supply fan in addition to measurement of the position of each terminal damper. Consequently, they have all the problems of the static pressure resetting inventions described above.
Accordingly, a need exists for a supply fan control strategy that can improve the part-load efficiency of supply fans in variable-air-volume systems without requiring the added cost of position measurements, without being sensitive to communications system failure, and being easy to tune.
In accordance with the present invention, a control strategy for a supply fan of a variable-air-volume heating, ventilating, and air-conditioning system comprises the supply fan, a fan modulating device, a static pressure sensor, an airflow sensor, and a controller coupled to the static pressure sensor, the airflow sensor, and the fan modulating device. The controller causes static pressure downstream of the fan to be reduced below a design static pressure when the airflow rate is below a design airflow rate.
The preferred method of reducing the pressure is to use a setpoint calculator that determines a static pressure setpoint as a function of the airflow rate. The relationship between the airflow rate and the setpoint could be linear, a polynomial function, or a relationship defined by a lookup table.
An alternative method of reducing the pressure is to calculate a loss coefficient that is the static pressure divided by a sum of a constant and the airflow rate raised to an exponent. The purpose of the constant is to ensure that the controller causes the fan to develop pressure at start-up, when the airflow rate is zero. The exponent should be preferably between 1.0 and 2.0. To limit the static pressure, a constant pressure mode is used in conjunction with the loss coefficient control mode. When the airflow rate becomes greater than the design airflow rate, the controller switches to the constant pressure mode, and the controller causes the pressure to be regulated to the design static pressure.
Accordingly, a primary object of the present invention is to provide a control strategy for supply fans of variable-air-volume heating, ventilating, and air-conditioning systems that can improve the energy efficiency at part-load conditions without requiring that the positions of terminal dampers be measured.
Another object of the present invention is provide a control strategy for supply fans of variable-air-volume heating, ventilating, and air-conditioning systems that can improve the energy efficiency at part-load conditions without being sensitive to a communication system failure.
Another object of the present invention is provide a control strategy for supply fans of variable-air-volume heating, ventilating, and air-conditioning systems that can improve the energy efficiency at part-load conditions without being sensitive to a failure of a terminal unit.
Another object of the present invention is provide a control strategy for supply fans of variable-air-volume heating, ventilating, and air-conditioning systems that can improve the energy efficiency at part-load conditions without being sensitive to a design flaw in which one or more terminal units is undersized.
Another object of the present invention is provide a control strategy for supply fans of variable-air-volume heating, ventilating, and air-conditioning systems that can improve the energy efficiency at part-load conditions and is easy to tune.
Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims, and detailed description of the invention.